Diving is the action where an individual immerses himself in water (sea, lake or river) to carry out marine-related activities, rescue operations, structure repairs, among others. Like any other activity, there are risks. Some might be present by the complexity of the work and others might be avoided by following certain safety rules. Nevertheless, diving in and of itself represents exposure to a dangerous disease called decompression sickness.
Decompression sickness (DCS) is a disease that occurs with the sudden loss of pressure which generates small bubbles. These bubbles (nitrogen bubbles), formed in blood and other tissues, are seen in divers who surface too quickly. DCS can occur when diving at approximate depths of 60 to 65 meters. As a safety precaution, a level of monitoring should be performed at a depth of 30-40 meters to identify the diver's condition and avoid unnecessary risks. Monitoring is important since this phenomenon may cause temporary paralysis, permanent injury or death. In 1878, a French scientist named Paul Bert, demonstrated the formation of bubbles in tissues and proposed the idea of a slow ascent to the surface. The symptoms associated with decompression can be divided into two types.
Type I is usually characterized by pain in the joints with mottling of the skin producing a red or purplish-blue tinge, fatigue, mood swings, and irritable behavior. Onset may be gradual and may be transient. Type II is characterized by central nervous system (CNS) spinal and cranial abnormalities often masked by pain distractions. In addition, some patients have symptoms such as: unusual fatigue, headache, and abdominal encircling. The locations of the above symptoms are illustrated in FIG. 1.
To avoid injury due to decompression sickness while diving, dive tables have been developed by the British biologist John Scott Haldane. These tables provide information about the speed of descent, ascent to the surface, and the waiting time for decompression stops, which allow bubbles formed to be dissolved. A typical diving table is the Diver table: No-decompression limits and group designation table, incorporated herein by reference. Since the occurrence of decompression sickness is correlated to bubble formation in tissues, some researchers began studies on the detection of bubbles using different methods.
The incidence of decompression sickness is a problem that continues to be studied since it is seen in divers that strictly follow dive tables. Research continues in order to determine the causes of decompression sickness; in this field, the main objective is the detection of bubbles. “All bubble detection techniques in some way fail to sample the entire population of bubbles that could potentially exist, for example, as a result of limits in sensitivity or resolution” (Leighton, 1994). One of the methods used to detect bubbles in this field of research is the Doppler method, mostly because the ease of operation and its relatively reduce cost. As the previous method, most other systems used today fail to detect the size and number of micro-bubbles prior to the occurrence of the decompression symptoms. Other methods have been developed using ultrasound as a detection tool for the presence of bubble.                1. Pulse echo-ultrasound: a technique in which pulses of ultrasound generated by a piezoelectric transducer are sent into the region to be studied (tissues), and echo signals resulting from scattering and reflection are detected and displayed. The depth of a reflective structure is inferred from the delay between pulse transmission and echo reception. Stephen Daniels in 1981 used an integrating pulse-echo technique where the number of pulse echo-ultrasounds in a scan of the tissue are counted and recorded in a preselected time interval as a signal. The stationary bubbles would be seen as an increase to the echo count and the non-stationary ones would contribute to the variability in echo count, therefore, the echo count gives an estimate of gas volume in tissues.        2. Doppler ultrasound: measures the shift in frequency of a continuous ultrasonic wave when the wave source and/or detector are in motion. Doppler is still the most sensitive technique available. In addition, a Doppler unit is far smaller and less expensive than most instruments required by other methods. The Doppler's ease of operation has aided in its popularity as a tool to examine divers after decompression.        3. Harmonic ultrasound: technique based on the principle of exciting a bubble with a low frequency signal and comparing it to the bubble resonance frequency. If acoustic energy from the bubble is scattered, it can be detected. The excited bubble will oscillate in a non-linear manner, emitting a wave with a doubled frequency.        4. Dual frequency ultrasound: uses a low frequency signal (“pump”) to excite bubbles with a known radius, which makes them resonate. A second-high frequency signal (“image”) is emitted. When it is incident on the resonating bubbles, a non-linear mixing occurs. This non-linear mixing causes results in a signal with the frequency of the “image” plus or minus the “pump” frequency.        
Techniques such as pulse echo ultrasound, harmonic ultrasound and dual frequency ultrasound are under study since Doppler bubble detection is useful for deciding whether a diver needs to receive hyperbaric treatment.